1. Field of the Invention
This invention relates to a vacuum processing system for performing substrate processing with plasma and in particular to a vacuum processing system having a structure for preventing plasma from arriving at the inner face of a vacuum vessel.
2. Description of the Related Art
Various vacuum processing systems for performing substrate processing with plasma are available. Hitherto, plasma etching and plasma CVD (chemical vapor deposition) systems have been known as equipments for manufacturing semiconductor devices, liquid crystal displays and the like.
FIG. 5 is a schematic view showing one example of such a conventional vacuum processing system. The vacuum processing system shown in FIG. 5 consists mainly of a vacuum vessel 1 provided with an exhaust channel 11, a gas introduction mechanism 2 for introducing a predetermined gas into the vacuum vessel 1, a power supply mechanism 3 for energizing the introduced gas for generating plasma, and a substrate holder 4 for placing a substrate 40 as an object in a position where the substrate is to be subjected to plasma processing.
The system in FIG. 5 carries a substrate 40 into the vacuum vessel 1 through a gate valve (not shown) and places the substrate 40 on the substrate holder 4. After evacuating air in the vacuum vessel 1 through the exhaust channel 11, the system introduces a predetermined gas by the gas introduction mechanism 2. Next, the system applies energy of high-frequency power and the like to the gas in the vacuum vessel 1 by the power supply mechanism 3 for generating plasma. Then, the system executes predetermined processing on the substrate 40 with activated species generated in the plasma.
When the plasma diffuses and arrives at the inner face of the vacuum vessel 1 in the vacuum processing system, it dissipates in that portion. Then, hitherto, a structure for setting a magnetic field along the inner face of the vacuum vessel 1 for preventing plasma from arriving at the inner face has been adopted.
FIG. 6 is a schematic sectional plan view for explaining the configuration of plasma arrival prevention magnets adopted for the above purpose. As seen in FIGS. 5 and 6, the plasma arrival prevention magnets 5 are plate-shaped permanent magnets placed so as to be in contact with the outer face of the vacuum vessel 1 and extend to the top and bottom. The magnets 5 are placed so that magnetic poles on the inner surface of the vacuum vessel 1 differ alternately, forming cusp magnetic field as shown in FIG. 6 along the inner face of the vacuum vessel 1.
Since charged particles in plasma are hard to move in a direction crossing a magnetic line of force, if plasma generated at the center of the vacuum vessel 1 diffuses to the peripheries, it is prevented from arriving at the inner face of the vacuum vessel 1. Thus, the plasma loss on the inner face of the vacuum vessel 1 can be prevented, maintaining the plasma in the vacuum vessel 1 at a high density for enhancing the processing efficiency for the object.
As processing continues in the conventional vacuum processing system, often a thin film is deposited on the inner face of the vacuum vessel. The thin film deposition on the inner face of the vessel is frequently observed in a thin film deposition system for executing thin film deposition processing such as CVD and an etching system for etching a thin film on a substrate.
When such a thin film deposited on the inner face of the vessel becomes thick, it peels off and becomes dust drifting in the vacuum vessel. If the dust adheres to the object, the quality of the vacuum processing may be seriously impaired. For example, if processing for manufacturing an integrated circuit is executed, a fatal circuit failure may be caused by dust adhering to the integrated circuit.
Since the conventional vacuum processing system of the type described above has the plasma arrival prevention magnets for preventing plasma from arriving at the inner face of the vessel, a thin film is comparatively less deposited on the inner face of the vessel. However, since the magnetic field distribution set by the plasma arrival prevention magnets is uneven, the thickness of the deposited thin film also becomes uneven. Resultantly, the following problems sometimes occur:
FIGS. 7(a) to 7(c) are illustrations of uneven thin film deposition on the inner face of the vessel. Here, for the description, the inner face of the vacuum vessel shown in FIG. 5 is converted into a plan. In FIGS. 7(a) to 7(c), the vertical axis denotes the film thickness or plasma density and the horizontal axis denotes the position on the inner face of the vacuum vessel.
As shown in FIGS. 6 and 7(a), magnetic lines of force 51 shaped like a small arc different in direction alternately are arranged along the inner face of the vacuum vessel by the plasma arrival prevention magnets 5. On the other hand, as described above, the plasma arrival prevention magnets 5 use the fact that plasma is hard to diffuse crossing the magnetic lines of force 51.
In this case, since the belly part of the arc-shaped magnetic line of force 51 is almost vertical with respect to a diffusion direction 52 of plasma to the inner face of the vacuum vessel 1, a sufficient plasma arrival prevention effect is produced. However, the direction of the magnetic line of force 51 crosses the plasma diffusion direction 52 at a small angle in the vicinity of the joint part of the arc, namely, the incoming or outgoing radiation point of the magnetic line of force 51 to the inner face of the vessel. Therefore, the plasma arrival prevention effect is weakened. This means that unevenness of the magnetic field vector causes the plasma arrival prevention effect to become uneven. Resultantly, the plasma loss on the inner face of the vessel in the vicinity of the incoming or outgoing radiation point of the magnetic line of force 51 increases and the plasma density in a direction along the inner face of the vessel (at positions equally distant from the inner face of the vessel) has an uneven distribution in which it is low in the belly part of the arc-shaped magnetic line of force and high in the joint part, as shown in FIG. 7(b).
In the joint part of the arc-shaped magnetic line of force 51 where the plasma arrival prevention effect is small, the inner face of the vessel is briskly irradiated with charged particles. As an ion assist method of applying a bias voltage to a substrate and irradiating the substrate with ions for accelerating thin film deposition is available in a thin film deposition process, thin film deposition on the inner face of the vessel is accelerated upon irradiation with the charged particles, and a thick film is deposited in a short time as compared with the belly part of the magnetic line of force 51. Resultantly, the film thickness distribution on the inner face of the vessel after execution of vacuum processing for a considerable time becomes a distribution where the film is extremely thick in the joint parts of the magnetic lines of force 51 and is thin (almost zero) in the belly parts, as shown in FIG. 7(c). The thickly deposited film in the joint parts easily peels off and causes harmful dust to occur as described above.
On the other hand, a plasma etching method is applied to removal of a deposited thin film. That is, for example, a fluorine-based gas such as carbon tetrafluoride and an oxygen gas are introduced into the vacuum vessel 1 by the gas introduction mechanism 2 and plasma is generated by the power supply mechanism 3. Fluorine-based activated species are formed in the plasma and the thin film is etched and removed by the brisk chemical action of the fluorine-based activated species.
In this case, since the etching progress depends on the activated seed generation amount, the etch rate distribution in the direction of the inner face of the vessel corresponds to the above-mentioned plasma density distribution. That is, many activated species are supplied to the belly parts of the magnetic lines of force 51 and etching well proceeds, but fewer activated species are supplied to the joint parts and etching does not sufficiently proceed. Thus, the etch rate becomes low in the joint parts requiring highly efficient etching and even if etching is executed for a predetermined time, a thin film is left in the joint parts. If an attempt is made to completely remove the thin film, the etching must be executed for a very long time, during which vacuum processing needs to be stopped, remarkably lowering the productivity.